1. Field of the Invention
The present invention relates to a top-loading monopole antenna apparatus for use in a communication system such as a mobile communication system or the like, and to a communication system and a mobile communication system which each have the same top-loading monopole antenna apparatus. In particular, the present invention relates to a top-loading monopole antenna apparatus including a short-circuit conductor which is electrically connected through a reactive element between a top-loading electrode and a grounding conductor, and to a communication system and a mobile communication system which each have the same top-loading monopole antenna apparatus.
2. Description of the Related Art
A top-loading monopole antenna apparatus has been widely and generally used as an antenna for use in a vehicle. The top-loading monopole antenna apparatus generally includes a linear antenna element, and the length thereof is often set to ¼ wavelength or ¾ wavelength. In the case of a frequency of 900 MHz for use in portable telephones, the ¼ wavelength is 83 mm, and the ¾ wavelength is 249 mm. In this case, the size thereof is too large as an antenna apparatus which is placed on a roof of a vehicle or on the inside of the vehicle. Accordingly, a top-loading monopole antenna apparatus as a low-profile monopole antenna apparatus has been developed.
FIG. 39 is a perspective view showing a structure of a top-loading monopole antenna apparatus of the prior art. The top-loading monopole antenna apparatus is constituted by including the following:
(a) a circular flat-plate-shaped top-loading electrode 11 (hereinafter referred to as an electrode 11);
(b) a circular flat-plate-shaped grounding conductor 14 that is provided so as to oppose the electrode 11 and has a feeding point 35 in the center thereof; and
(c) a linear conductor element 12 that electrically connects the center of the electrode 11 with the feeding point 35; and
(d) a short-circuit conductor 13 that electrically connects a point on the electrode 11 which is different from the center of the electrode 11 with the grounding conductor 14.
In this case, a central conductor of a coaxial cable 30 for feeding electric power or transmitting a RF signal is electrically connected with the feeding point 35, and a grounding conductor of the coaxial cable 30 is electrically connected with the grounding conductor 14.
The top-loading monopole antenna apparatus of the prior art is constituted by connecting the circular flat-plate-shaped electrode 11 with a top portion of a top-loading monopole antenna apparatus. By employing the circular flat-plate-shaped electrode 11, the top-loading monopole antenna apparatus, for which a height (length) of 83 mm was required at ¼ wavelength in the case of a frequency of 900 MHz, is allowed to have a low-profile configuration of a height of 30 to 40 mm.
Next, the problems of the prior art, which are attempted to be solved by the present invention, will be described hereinbelow.
A first problem of the prior art relates to impedance matching between the antenna apparatus and the coaxial cables 30 for feeding electric power or transmitting a RF signal. When the number of the short-circuit conductors 13 is increased, the top-loading monopole antenna apparatus can control the input impedance of the antenna apparatus. However, this leads to such a problem that the resonance frequency of the antenna apparatus, and then, failing in achieving impedance matching at a lower frequency.
A second problem of the prior art relates to the size of the circular flat-plate-shaped electrode 11. If the top-loading monopole antenna apparatus is made to have a low-profile configuration, the size of the circular flat-plate-shaped electrode 11 is then required to be increased. This is undesirable from the viewpoint of size reduction. The reason for the need to increase the size of the circular flat-plate-shaped electrode 11 will be described hereinbelow with reference to FIG. 40, which is a longitudinal sectional view showing currents flowing in the top-loading monopole antenna apparatus of FIG. 39.
Referring to FIG. 40 showing the top-loading monopole antenna apparatus of FIG. 39, a current 21 flows in the linear conductor element 12 from the linear conductor element 12 toward the circular flat-plate-shaped electrode 11, and the current 21 flows in the electrode 11 from the center portion of the electrode 11 toward the edge portions thereof as indicated by the current 22 so as to be parallel to the grounding conductor 14. In this case, the electric field distribution of the antenna apparatus can be considered as a sum of electric fields which are caused by the current 21, the current 22 and an image current 23 which is reverse to the current 22. The image current 23 is not an actually existing current but is a current for obtaining an equivalent electric field distribution assuming that the grounding conductor 14 does not exist in the monopole antenna apparatus. In this case, a distance between the current 22 and the image current 23 is double the distance between the circular flat-plate-shaped electrode 11 and the grounding conductor 14. That is, it can be assumed that the image current 23 corresponding to the current 22 flows axisymmetrically with respect to the grounding conductor 14.
Making the top-loading monopole antenna apparatus have a low profile is to shorten the distance between the circular flat-plate-shaped electrode 11 and the grounding conductor 14. At this time, the distance between the current 22 and image current 23 is also shortened. The electric field that is caused by the current 22 and the electric field that is caused by the image current 23 are reverse to each other, and therefore, mutually canceling electric fields increase as the distance decreases. Due to compensation for the canceled electric fields, the current 21 flowing in the linear conductor element 12 and the current 22 flowing in the circular flat-plate-shaped electrode 11 increase. In this case, in order to maintain the input impedance constant, it is necessary to provide an increase in the resistance component for the increase in the current. Therefore, to increase the resistance component, the size of the circular flat-plate-shaped electrode 11 is increased.
A third problem of the prior art relates to the usable band. If the height of the antenna apparatus is lowered, then the bandwidth is narrowed. There is such a problem that the bandwidth used by the application to use the antenna apparatus is predetermined, and this leads to a limitation on the ability to lower the height of the antenna apparatus.
A fourth problem of the prior art relates to providing an antenna apparatus in a vehicle. An antenna apparatus which is provided in a vehicle should preferably have, in particular, a compact configuration. If an ordinary top-loading monopole antenna apparatus is made to have a low-profile antenna configuration as described above, then the size of the circular flat-plate-shaped electrode 11 increases, and the required size of the grounding conductor 14 also increases. It is often the case where a sufficient size of the grounding conductor 14 cannot be secured in a vehicle, and accordingly, there is also a limitation on the height of the antenna apparatus made to have a low-profile configuration. The height of the top-loading monopole antenna apparatus of the prior art becomes 30 to 40 mm due to the restriction on the size of the grounding conductor 14, and it has been unsuitable for use in a vehicle.